Bridge Building and Testing
Laser cutting
For the laser cutter workflows look here. The only caveat is that the big laser, currently has to be manually focused. To do so, flip down the manual focus part, then move the head over where you are cutting, and move the bed upward until the manual focus probe is slightly touching the material. Then flip up the manual focus probe, and you are good to start your cut. To include your file in the laser cutter, go to Corel draw then select print, and from there you can change the placement and settings of the laser cutter, and can start your cut. There were no major differences in how my cut turned out relative to the digital design. For this project I did not implement the kerf value, as it is a negligible value, which can be found here.
Part Processing
To make the bridge, first we laid out sheets to scale of the bridge model on a sheet of cardboard and glued it down. This allowed for more ease of use in the future when actually assembling the bridge.
To test if the parts were the right size, we first cut one file of the gusset plates, but they did not fit the footprint on the schematic. To make it fit, we scaled up up by 10 percent in Corel Draw, and then it fit correctly
In order to assemble the bridge, we used score marks on the tubes, folded them ion the score lines, then used super glue to glue them together. For certain pieces we had to cut them down to a smaller size, such as the end posts and some of the tension bars. We checked these measurements with the layout we created previously, which was to scale of the actual bridge.
With the required parts, they are all found here within my file I used for laser cutting, this consists of the tension bars, as well as various tube lengths and dimensions made for the specifications of the west point bridge building document.
Issues with part processing
Some issues that were encountered was that some pieces were of incorrect length, so Ryan Kim, Adam Stone and I redesigned the pieces so that they fit correctly. There were also a few issues here with some of the members glued, in that they were warped, so I had to recut new members and glue them again.
Bridge construction
To create the actual bridge, we cut our pieces out of chip board, the material commonly used in cereal boxes. When using the laser cutter, we used the cardstock cut and score settings on the big laser cutter, and on the medium laser cutter we used the carboard settings for cut, and then 15 power 100 speed for the scoring when we had to use that laser cutter. We followed the part processing assembly for the parts, then used the layout we created to assemble the bridge accurately. Adhesion between pats was achieved through super glue and the laser cut chipboard material. The exact details for the assembly can be found in the West point bridge building textbook which can be found here. Some potential issues with the bridge could be the tension bars on the bottom of the bridge, some of them are not very tense which could lead to some issues with the how well that portion of the bridge handles force. The prevention of this issue could be achieved through more carefully gluing down the pieces and making sure that they are very precise in their exact placements.
Testing the Members
To test the various parts of the bridge we first laser cut out some members similar to those on out bridge. Following the West Point bridge testing document, I specified these various pieces to laser cut. Below is a picture of the full design in Corel Draw
Once I had designed all the pieces, I laser cut them and glued them all together similar to how I did with the previous tubes when building the actual bridge. Once the tubes were glued, I used the longer end cap pieces on the 10 X 10 mm tubes, and the shorter on the 6 X 10 tubes. Once I had all the tubes glued together, I glued a square to each end of the strips, and repeated until I used up all of the strips.
Then I set up the spreadsheet that can be found at the bottom of this page in order to manage the values I received from testing each piece using the "bridge testing machine". To set up the tests, I began with the smallest 10 X 10 tube, and place it beneath the lever and the fulcrum. Then I measured the distance from the center of the piece to the pivot point, and then the initial measurement from where the weight was to the pivot point. Once I had these values I began to place weight on the member until it broke or was unable to support more weight. Then I measured the weight and added taht value to the spreadsheet.
Then after the tubes, I used a similar procedure with the strips. I clamped the strips down on the opposite side of the pivot point as the weight and then measured that point. After I had the measurement I began to add weight, and again, after it broke weigh that and place that value into the sheet.
IMG_2274.MOVVideos by Bridgett Yu and Ginny Foster
IMG_2325.MOVBridge Data Values
The "Break force" tab for each test used the equation: ((Distance from weight)*weight)/ distance from member This equation was plugged into the spreadsheet so it would automatically update whenever one of the values are changed. The "distance from member" column was the measured distance from the fulcrum to the piece being tested. The "distance from weight" column represents the distance from the weight to the fulcrum. The differing weight values of each type of member could be explained by how strongly they were glued together, or the placement/orientation of the pieces on the testing machine, likely led to the variability within the data.
Some potential weak points of the bridge are definitely the bottom tension bars, because they are taking a large amount of force, and are likely not going to stand for so long due to the amount of force that can be place upon them.
These are the different sizes of tension bars I will be using for the tests
This is the gusset plate file after I modified and changed the size of it.
These are the different tube variations for the tests
Bridge Testing
To test the bridge, we used a similar method to that which we tested the bridge using the method of joints. To achieve having the load in just 6 points, we just placed Legos on 6 points to ensure those were the only places which the load could apply force
Then once the Legos were in place, we placed books on the bridge to add force. We began with just 3500 g, then placed on 5003g when I saw the bridge was stable, and our bridge managed to hold both loads well, with no signs or sounds of breaking. However, based off the information of the bridges I saw others having built breaking, i could see that a common failure point was the tension bars, so a potential improvement would be widening them to increase their factor of safety, and potential load holding.
IMG_0370.MOVConclusion
To create the bridge, I used Corel draw to create the files needed for the members, and then sued the laser cutter, for which the workflow can be found above, to cut those pieces out. I used super glue to glue the tubes together, then the template I made to place all the pieces of the bridge out, and to begin assembly, again using super glue and the instructions from west point. I made the main bridge, as well as some pieces to test, of which there were two types of tubes in 3 different sizes each and 3 different widths of tension bars. The data a came up with was heavily skewed and clearly had some variabilities to it unfortunately. From this project I learned the basics of how bridges are designed and tested, it was very interesting to see exactly where different types of bridges had strengths, and were they had weaknesses. Accounting for tension and compression in a bridge are also very important, because the design dictates the materials used, and how strong different pieces need to be to make the bridge stable and safe. To make the bridge we designed stronger, it would be wise to increase the width of the tension bars, and also to go slowly when building to limit human error within the bridge.